The present invention relates generally to reducing the shrinkage of resist materials during the fabrication of semiconductor or other devices and, in particular, to techniques for reducing resist shrinkage that can occur during etch processes.
Lithographic processes are used to manufacture, for example, semiconductor devices, integrated optics and photomasks. Such processes employ various energy sources to create a relief image in a film of resist material applied on a substrate or wafer. Energy sources typically used in the lithographic processes include light and electron beam radiation.
Lithographic processes may use resists that contain polymeric materials, some of which include a protective group attached to the polymer. The protective group facilitates the formation of the resist material and may alter the solubility characteristics of the polymer.
Certain protective groups, when attached to the polymer, function as moieties that render the polymer relatively insoluble in alkaline solution. In lithographic processes, the protective groups or moieties are removed upon irradiation and baking of the polymer film in the presence of a radiation-induced acid.
In some lithographic processes, the acid is provided by a photo-acid generator (PAG) that produces an acid moiety upon irradiation with deep ultraviolet (UV) light. An exemplary PAG/acid sensitive polymer combination includes onium salt as the PAG and a polymer such as poly(4-tertbutoxycarbonyloxystyrene) that has a reactive substituent such as a tert-butocarbonyl protective group. Alternatively, the polymer itself can provide an acid functionality upon chain scission, thereby eliminating the need for an added PAG. An exemplary acid sensitive polymer that provides an acidic functionality upon radiation-induced chain scission includes poly(4-tertbutoxycarbonyloxystyrene-sulfone). Such polymers are referred to as chemical amplification (CAM) polymers since the production of one molecule of acid by radiation induces a reaction in multiple reactive substituents in the acid sensitive polymer.
As a result of the removal of the protective groups or moieties, the polymer becomes more soluble in alkaline solutions. After a substantial percentage of the protective groups or moieties have been cleaved from the exposed polymer, the polymer in the exposed region(s) of the film is substantially more soluble in an aqueous alkaline developing solution.
Acid generally is not generated in the non-exposed region(s). Therefore, the protective groups or moieties are not cleaved from the polymer in those region(s), and the resist material in those regions is not as soluble in an alkaline solution. If an alkaline solution is used to develop the image projected onto the resist, the material in the exposed region(s) is dissolved by the developer solution, while the material in the unexposed regions is not. A positive tone image or pattern is, thus, developed that corresponds to the image projected into the resist material.
In some device fabrication processes, the resulting resist pattern is used to define a mask for an etch process, such as a reactive ion etch (RIE) or other dry etch process. During such etch processes, however, the previously-unexposed resist pattern may be exposed to radiation in the form of a plasma or light, as well as energy in the form of heat. Depending on the wavelength(s) of the radiation in the etch chamber, acid may be generated in the resist. If the temperature in the etch chamber is sufficiently high, the acid can cleave the bonds of the protective groups or moieties in the resist, which can result in shrinkage of the resist pattern. Shrinkage of the resist during the etch process can result in distortion of the etch pattern and in a loss of image quality, so that the resist pattern is not properly transferred to the underlying layers of the device. Problems resulting from shrinkage of the resist can be particularly acute for wave-shaped, curve-shaped or S-shaped resist patterns. As illustrated in FIGS. 1A through 1D, shrinkage of a curve-shaped resist pattern can result in the top of the resist sloping away from the bottom of the resist. Thus, the desired pattern is not transferred properly to the underlying layers during subsequent etch processes.